The field of this invention relates to incandescent mantles as employed with fuel-burning devices to produce illumination, such as portable fuel-burning lanterns.
During the Gaslight Era, extending from the latter part of the 19th century into the early part of the 20th century, incandescent gas mantles were products of major commercial importance. During that period, there was intensive research and development to provide mantles of better illuminating power with respect to the amount of gas being burned, and also to provide mantles which were adequately strong and durable in use. The history of the manufacture of incandescent gas mantles is described in The Rare Earth Industry, Martin editor (Crosby, Lockwood & Son, London, 1918), Chapt. I, pages 15-28.
The first mantles were made of oxides of zirconium, lanthanum, and yttrium. These mantles had poor strength and durability and inadequate lighting power. Thorium oxide mantles were an improvement, but did not become commercially successful until around 1893 when Welsbach developed and patented a thorium oxide mantle containing around 1% cerium oxide. The cerium oxide promoted the light emission of the thorium oxide with a radiation peak around 0.92% cerium oxide. (See FIG. 5, page 16, The Rare Earth Industry, cited above.) Welsbach's U.S. Pat. No. 563,524 of 1896 covering his thorium oxide-cerium oxide mantle. The Welsbach mantle as used commercially also contained a small percent of magnesium oxide.
Although many attempts were made to develop a better mantle, the Welsbach mantle became the standard mantle for gaslight illumination. The preference for the Welsbach mantle has continued to the present day with respect to its use in portable incandescent lanterns. One widely used commercial formula for lantern mantles is 99.1% ThO.sub.2, 0.6% CeO.sub.2, and 0.3% MgO. In its application to portable lanterns, however, this version of the Welsbach mantle has provided problems in both manufacturing and use.
The Th.sup.232 isotope which comprises the principal isotope component of thorium is the parent of the thorium decay series. It causes the ThO.sub.2 to be mildly radioactive. Although the amount of radioactivity in the finished Welsbach mantles is harmless and miniscule, special handling precautions are necessary to protect manufacturing personnel when large quantities are used. These procedures are complex and make processing difficult and costly.
Although the illuminating power of the Welsbach mantle is high, its strength and durability are not entirely satisfactory. These weaknesses are especially notable in portable lanterns. Portable lanterns are subject to shocks in handling, which jar the mantles after they have been fired and are in a relatively fragile condition. For lacquered mantles as manufactured in the United States, the attachment or head portions of the mantle sacks have been hardened by applying a solution of aluminum nitrate and other metal salts. This protects the mantle heads but cannot be used for the mantle bodies because the hardening metals are not luminescent. In use the mantles tend to rupture below the attachment heads. Greater tensile strength and greater durability has been needed.
In the commercial manufacture of Welsbach type mantles for lanterns, tests have been developed for the purpose of attempting to standardize tensile strength and/or durability properties. In one of these tests, referred to as the "Bump Test", test specimens of a production run of the mantle sacks after firing and cooling are subjected to repeated bumping shocks. It has been found that the average durability of the fired mantles on the basis of the Bump Test is lower than desirable, and that the Bump Test survival counts vary considerably despite standardized manufacturing. Mantles from some production runs break after as little as 50 to 100 bumps, while others last for as long as 600 to 800 bumps. The cause of this variability is not known. Prior to the present invention no means has been known for improving the tensile strength and durability of the mantles, or for providing improved consistency of durability in manufacturing.
As will subsequently be described in greater detail, the improved mantle of the present invention contains yttrium oxide as a major ingredient. Mantles composed primarily of yttrium oxide are not known to have been previously used commercially although some of the early commercial mantles are understood to have contained minor amounts of yttrium oxide. The patent art in the period from 1890 to 1910 includes patents referring to the use of yttrium oxide in mantles in admixtures with other oxides. The following references are believed to be representative:
Hicks Pat. No. 703,064 of 1902; PA1 Welsbach Pat. No. 399,174 of 1889; and PA1 Welsbach Pat. No. 359,524 of 1887.